Air bag containers, also called air bag canisters, are housings for motor vehicle safety airbags. Airbags are a key passenger safety feature. When called upon, they have to work perfectly and they only have one chance to do that. Airbag systems undergo destructive testing as they cannot be individually tested to determine whether each one works properly before installation in the vehicle. Manufacturers have to be sure that the airbag they build will work as intended. To do that, they need to design and build systems with known safety margins that exceed the performance requirements. Airbag safety devices are complex systems where every component contributes to a safe deployment. The more accurately a component can be made and the narrower the tolerances, the more assured is the performance. Airbag containers or airbag housings are passive components that play a role as vital as the more familiar key components, which are the gas generator, airbag and the cover. The housings are passive as they have to remain as they were before deployment. If the housing experiences unforeseen deformation under the force of the explosive charge and if it starts to crack and split, the airbag will not deploy as designed with potentially disastrous consequences for any vehicle occupant. Worse still, if the housing undergoes drastic and brittle failure, fragments of the housing can cause serious harm to the driver or passenger who the system is intended to protect. In view of this, air bag containers must have a high dynamic burst pressure resistance, because air bag containers are subjected, when deployed, to a sudden and intense internal pressure.
The material currently applied for air bag containers are mainly metal but also plastic airbag containers are more and more applied in view of the reduced weight. Air bag containers have to work in extreme conditions, demonstrating that they can perform under all conditions; at extreme low temperatures, down to −35° C., and high temperatures, up to 85° C. At low temperatures, the risk is that the plastic housing will crack and fail by brittle fracture, leading to inadequate or failed airbag deployment and splinter generation. At high temperatures, the plastic materials have to retain sufficient strength or fracture will occur with similar consequences or even worse consequences. Only dedicated developed materials will survive these extreme conditions. Impact modified, reinforced polyamide-6 compositions are currently applied for air bag containers requiring high dynamic burst pressure. These compositions can be formulated to have very high impact performance at low temperature, thus avoiding brittle failures, while retaining more than enough strength to perform at high temperatures. Impact modified, reinforced polyamide-6 compositions can be formulated such that an air bag container consisting of such composition possess a dynamic burst pressure higher than 1.6 MPa (measured at −35° C.). Another requirement however is a cost effective production process for such moulded parts, among other things short cycle times. A disadvantage of such impact modified polyamide compositions that can advantageously be used for moulding air bag containers requiring high dynamic burst pressure, in particular at low temperature, and high strength, in particular at high temperature, is that the melt flow of such compositions is relatively low, resulting in longer cycle times of the moulding process and thus in a slower and more costly moulding process. The cycle time of a moulding process is usually referred to the time span that starts when the mould closes and ends when the mould opens and the part is ejected. The cycle time is usually dominated by the cooling of the part inside the mould cavity. The cycle time can be calculated using tcycle=tclosing+tinjection+tcooling+tejection, where the closing time tclosing, the injection time tinjection and ejection time tejection only last from a fraction of second to a few seconds and the cooling time tcooling dominates the process. However, the addition of components to the polyamide composition intended to increase the melt flow of the composition usually results in a significant decrease of the dynamic burst pressure of the moulded part. Thus, one difficulty with such compositions is the achieving and sustaining of high melt flow polyamide compositions without significantly compromising the dynamic burst pressure performance of the air bag container, in particular at low temperature.
The invention now aims to provide an air bag container with a high melt flow, but not at the expense of the desired property of high dynamic burst pressure, in particular at low temperature. More in particular, the aim of the present invention is a uniquely favourable balance of high dynamic burst pressure, in particular at low temperature, and short cycle time.
The invention relates to air bag containers consisting of an impact modified polyamide composition having a Melt Volume-Flow Rate (MVR) higher than 25 cm3/10 minutes (275° C./5 kg).
Surprisingly it has been found that the dynamic burst pressure performance of impact modified polyamide composition could be retained to a high degree and at a sufficient high level, whilst at the same time major cycle time reduction can be achieved. More in particular, dynamic burst pressures up to and even higher than 2 MPa, measured at −35° C., could be obtained, using a polyamide composition with such a high melt flow.
Preferably, the dynamic burst pressure at −35° C. is higher than 1.6 MPa, more preferably higher than 1.7 MPa and even more preferably higher than 1.8 MPa.
It has been found that the Melt Volume-Flow Rate of the polyamide composition can even be increased to a value higher than 30 cm3/10 minutes, even to a value higher than 40 cm3/10 minutes (275° C./5 kg). The Melt Volume-Flow Rate of the polyamide composition is generally lower than 100 cm3/10 minutes (275° C./5 kg) and is mostly lower than 80, because at a Melt Volume-Flow Rate of the polyamide composition higher than 100 it is almost impossible to keep the dynamic burst pressure at −35° C. at a sufficient high level.